1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies and, more particularly, is directed to a spring design for a grid of a nuclear reactor fuel assembly. Specifically, the invention relates to an improved spring and dimple configuration for a retention plate of a grid strap.
2. Description of the Related Art
In most pressurized water nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids that are spaced axially along the fuel assembly length and are attached to a plurality of elongated thimbles of the fuel assembly. The thimbles typically receive control rods or instrumentation therein. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids, as well known in the relevant art, are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a multiplicity of roughly diamond-shaped cells which individually accept the fuel rods and thimbles therein. The straps are configured such that the cells each include a plurality of relatively resilient springs and a plurality of relatively rigid dimples, the springs and dimples being formed into the metal of the interleaved straps and protruding outwardly therefrom. The springs and dimples of each cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid.
One type of prior art strap is depicted generally at the numeral A1 in FIG. 1 and the numeral A2 in FIG. 2. Straps A1 and A2 are identical but inverted views of identical straps, and thus include identical components. The specific components of the straps A1 and A2 thus will not be differentiated herein. The strap A1 is in a "slots up" orientation and the strap A2 is in a "slots down" orientation.
The straps A1 and A2 each include an elongated strap body C that is formed out of a strip of a sheet metal that is suited to a nuclear environment. The strap body C is formed with a plurality of slots E that are parallel with one another and that extend midway into the strap body C. Each strap body C includes a plurality of retention plates G thereon that are defined on the strap body C between adjacent slots E. Each retention plate G includes a free portion 1 that terminates on opposite sides at the slots E and a connected portion K that is defined within the strap body C and that terminates at imaginary sides that are aligned with the slots E.
A spring M and a pair of dimples O are formed in each retention plate G by stamping and cutting appropriate holes into the strap body C or by other known methods. Each spring M includes a first spring ligament Q, a second spring ligament S, and a spring contact plate U. The spring M is oriented at approximately a 45.degree. angle with respect to a longitudinal axis that extends through the strap body C and that is substantially perpendicular with the slot E.
The first spring ligament Q extends between the connected portion K and the spring contact plate U. The second spring ligament S extends between the spring contact plate U and the free portion 1. The retention plates G are each configured such that the spring M, and particularly the spring contact plate U, protrudes outwardly from the strap body C in a direction generally out of the plane of the paper of the paper of FIGS. 1 and 2. Similarly, the dimples O are configured to extend outwardly from the strap body C in a direction generally into the plane of the paper of FIGS. 1 and 2. The springs M thus protrude outwardly from the strap body C in a direction opposite the dimples O.
When the straps A1 are interleaved with the straps A2 by engaging the slots E of the straps A1 with the slots E of the straps A2, a pair of springs M and two pairs of dimples O protrude into each cell. The springs M, with their first and second spring ligaments Q and S, are generally longer in length than the dimples O, and thus have a smaller spring constant than the dimples O. As such, the springs M are relatively resilient in comparison with the dimples O, which are relatively rigid.
When the fuel rods are inserted into the cells and are in a compressive relation with the springs M and the dimples O protruding into the cell, the springs M, and to an extent the dimples O, are deflected away from the fuel rods and thus impart a residual retaining force against the fuel rods to hold the fuel rods in a given position. It can be seen, however, that since the first spring ligament Q connects with the connected portion K which terminates at an imaginary line within the strap body C, and the second spring ligament S connects with the free portion 1 which terminates at the slots E, the first spring ligament Q will have a higher spring constant and thus will be less resilient than the second spring ligament S. Such differential resilience has a tendency to impart a rotational force to the fuel rod. Such rotational forces have the effect of applying a torque to the fuel rods and of abrading or fretting holes into the fuel rods. Such wear on a fuel rod is preferably avoided due to the potential of releasing radioactive material into the primary cooling loop of the reactor.
In order to reduce the wear on fuel rods, it is thus desired to provided an improved spring design having an even deflection profile in directions both parallel and perpendicular with the longitudinal axis of the strap body. It is also desired to provide an improved spring design that will reduce the surface stress at the interface between the spring contact plate and the fuel rod. It is additionally preferred to configure the new spring design to minimize the abrasion that occurs to the fuel rod when the fuel rod is initially inserted and finally removed from the grid made up of the straps A1 and A2.